No, seriously, why should anyone hire a designer? I am one, but I don't do it for the money, believe me! If I didn't adore garden design, I wouldn't be doing it.
I am sure I'll come back to this again and again. First off, let me note that there's a difference between hiring someone whose job it is to design for your project and act as your advocate vs. someone who's job it is to buy (and markup) the materials, install the elements, maintain it, or do any other sales-affiliated work. This 'sales' person could be an ambitious gardener, a design-build company, or a straight-up contractor. There is overlap everywhere, as soon as I say that contractors can't design, someone will send me a link to a contracting company that hires professional designers.
If knowing that the person you hire is interested only in the safety and aesthetics of the project, and is advocating exclusively on your behalf, you should hire a designer. It is your designer's job to be cost-conscious and to listen to what you need. I have worked in firms that charged design fees based on construction cost - I always hated that. There isn't a design fee difference in designing a patio out of bluestone vs. limestone or brick. If your fence is Pine or Redwood, the details take the same amount of time to draw. However, those folks in the 'sales' business have a different profit margin based on the cost of the materials you select.
I worked for a while as a designer at a retail nursery. It was my job to design plantings for people's homes. I was expected to use the plants currently in the nursery and not ask for a special order. If the client bought their plants from the nursery, the design fee (a whopping $50.) was waived! In addition, my clients got planting plans that were specific to the time of year that they hired us in. If someone came in during summer, they got summer season plants. The good part was that I could actually pull aside the materials I was proposing and set them aside for the client to see in real life. Today, I rely on digital images, sketches, and visits to multiple nurseries instead.
So why hire a designer? The expectation is that the design work will be well-rounded, fitted to your budget, lifestyle, and aesthetics. The designer should help you make sound decisions based on form and function, not their own bottom line.